This invention relates to Metal Oxide Silicon Field Effect Transistors (MOS-FETs) and in particular, to methods for stabilizing drifts in threshold voltage in MOS-FET structures.
Drifts in threshold voltage of MOS-FETs during normal use can constitute a potentially serious threat to the reliability of many of the MOS integrated circuits produced today. One well known cause of such drifts is the mobile charge contamination (particularly sodium contamination) which manifests itself in an increase in the flat band voltage (V.sub.FB) upon positive bias-temperature aging. The stability of the threshold voltage has been controlled by the use of clean processing techniques including dry HCl oxidation, an intermediate gettering step, and a final passivation layer. Another important source of such threshold voltage drifts especially in, but not limited to [111] silicon-silicon dioxide structures, is the so-called slow-trapping instability. It produces a negative shift in V.sub.FB and therefore in the threshold voltage, upon negative bias-temperature aging. It also produces a positive shift in V.sub.FB for positive bias-temperature aging, but these shifts are generally smaller than those produced by negative bias-temperature aging. The cause of slow-trapping instability is not completely understood. It has been attributed to hole-trapping in the oxide or to field induced dissociation of additional silicon bonds in partially ionized silicon atoms near the silicon-oxide interface of an integrated circuit.
The publication entitled "Stabilization of MOS Devices" in Solid State Electronics, Volume 10, pp. 657-670 (1967), discusses the problem of slow-trapping instability in MOS devices and teaches the use of annealing the devices in dry hydrogen or helium at temperatures between 1,000 and 1,200 degrees C. to negate the effect slow trapping has on threshold voltage changes. A problem with using temperatures over 1,000 degrees C. is that silicon dioxide tends to get reduced by hydrogen at these temperatures. This causes an increase in the oxide fixed charge and an undesirable change in the threshold voltage of the MOS structure. In addition, many of today's MOS integrated circuits (e.g., p-channel 1024 bit RAMs) use boron doped polysilicon, and fabrication temperatures in excess of 1,000 degrees C. cause boron to diffuse into the silicon dioxide. The boron diffusion is enhanced by presence of hydrogen in the ambient. The presence of boron at the oxide-silicon interface reduces the oxide fixed charge and thereby changes the electrical characteristics of the device.
It would be desirable to be able to limit the degrading effect slow trapping has on threshold variations in MOS structures without adding undue complexity to the semiconductor fabrication process.